Field of Invention
The present invention relates to LED, and more specifically relates to an LED driver.
Description of the Related Art
The LED (Light-Emitting Diode) lamps are widely used in a variety of electronic applications due to LED lamps have significant advantages, such as long life time, small size, and high efficiency. In general, the LED lamp includes a plurality of LEDs, and the LEDs are connected to each other in series as an LED string. The LED string is driven by the rectified AC (Alternating Current) voltage. The voltage level of the rectified AC voltage always changes with respect to time. Therefore, the voltage level of the rectified AC voltage isn't always higher than the summation of the forward voltages of all LEDs, therefore some LEDs aren't luminous. Therefore, the utilization rate of the LEDs is weak.
In addition, most LED systems comprise a dimmer, such as TRIAC dimmer, which is used to adjust the brightness of the LED lamps. The TRIAC dimmer is triggered every half of AC cycle. While it is trigged, the current flowing through it should be kept higher than a threshold current for the remaining half AC cycle. The threshold current is called holding current.
Accordingly, the present invention provides an LED driver for increasing the utilization rate of LEDs of the LED string when the input voltage is lower and sinking a bleeding current in order to keep the current flowing through the dimmer higher than the holding current.